1. Field Of The Invention
This invention concerns an improved thermocouple for the temperature measurement of hot gases produced by the partial oxidation of ash-containing liquid hydrocarbonaceous and/or solid carbonaceous fuels. More specifically, it concerns a high temperature slag resistant thermocouple sheath.
2. Description Of The Prior Art
Thermocouples are used for measuring temperature in the well-known high temperature partial oxidation system for producing synthesis gas, reducing gas, and fuel gas. Thermocouples are pairs of insulated wires of dissimilar metals which are connected at both ends. When the two junctions of the wires e.g. T.sub.2 and T.sub.1 are at different temperatures, a difference in electrical potential exists between them. The thermal electromotive force (e.m.f) is a measure of the difference in temperature between T.sub.2 and T.sub.1. A voltage-measuring instrument placed in the thermocouple circuit will measure temperature. For example, a sensitive high-temperature thermocouple consisting of platinum-platinum 10% rhodium will generate an e.m.f. of 9.457 millivolts and 17.339 millivolts when the differences between the hot junction and the cold junction in degrees F are respectively 1800 and 3000. Other pairs of metals e.g. chromel-alumel and iron-constantan are used to measure lower temperatures.
A corrosive atmosphere prevails during the operation of a high temperature partial oxidation gasifier. Any unprotected thermocouple in this atmosphere will be attacked and rendered useless, and especially when iron is present in the reaction zone. Various metals, alloys and refractory materials have been used in the past to form thermocouple protective sheaths. However, these materials were found to be unsuitable for long-time operation in partial oxidation systems where the temperature at various locations exceeds 1000.degree. F. Impurities in liquid hydrocarbonaceous fuels and solid carbonaceous fuels, especially iron, readily attack thermocouples. Some noble metals e.g. platinum were found to be a sink for iron. Daily replacement of thermocouple sheaths made from these materials was not unusual. Iron-containing materials e.g. iron oxide are a major additive component in the gasification of fuels such as heavy liquid hydrocarbonaceous fuel and/or solid carbonaceous fuel that already contain iron, vanadium and nickel impurities. See related U.S. Pat. No. 4,668,428, which is incorporated herein by reference.
In coassigned U.S. Pat. No. 4,776,705, which is incorporated herein by reference, a thermocouple is enclosed in a noble metal protective sheath which in turn is inserted inside a multi-segment refractory thermowell. An annular space formed between the outside surface of the sheath and the inside surface of the thermowell is continuously purged with an oxidizing gas or gaseous mixture. Gasification products from the residual slag, which are normally in a form that react with and destroy the thermocouple wires, are neutralized or oxidized within the annular space by reaction with the purge gas mixtures. In contrast, advantageously by the subject invention, the aforesaid purge gas is eliminated, during the partial oxidation reaction, temperature measurments are more accurate, and the design of the thermocouple is simplified at a significant cost savings.